1. Field of the Invention
The present invention relates to a tubular valve guide with a bore for a metal valve shank of an internal combustion engine.
2. Description of the Prior Art
Valve guides are subject to extremely high thermal and mechanical stress. As both the guide and valve shank normally consist of metals, the heavy stress may lead to the occurrence of welding, resulting in imprecise operation of the valves. This impairs the functioning of the engine and reduces its life-span. Therefore, the selection of the materials for the manufacture of valve guides and valve shanks is of special importance. DE-OS No. 30 49 154 proposes to replace the known metallic valve guides of Niresist, tin bronze, aluminum bronze or cast iron by a tubular valve guide with a nitrided inner surface and thereby to reduce the wear. However, this is disadvantageous because only the surface of the bore is hardened, but the danger exists that after this hard layer has eroded, much faster wear occurs. Also, this proposal is less suitable for the special problem existing in the blow-off valve of an exhaust gas turbocharger because, in this case, the danger of oxidizing, i.e., the formation of fuel residues, persists. This danger is known to exist especially for metal parts exposed to combustion gases, and, more particularly, in the blow-off valve of an exhaust gas turbocharger, where there are relatively long standstill times between the individual cycles. Inlet and outlet valves or their valve guides, are less endangered in this respect because they are in operation practically without stopping and therefore, residues form in the valve guides to a much lesser degree since minor residues are removed by the continual working of the valves.
Valve guides of silicon carbide and silicon nitride are known from Japanese patent publication No. 55-78119. Silicon carbide has a heat conductivity of approximately 85 to 90 W/(m.multidot.K) and a thermal linear expansion coefficient of approximately 4.4.times.10.sup.-6 K.sup.-1, while silicon nitride has a heat conductivity of 30 to 40 W/(m.multidot.K) and a thermal linear expansion coefficient of 3.2.times.10.sup.-6 K.sup.-1. See, for example, the brochures of the Feldmuhle Aktiengesellschaft. However, in spite of these favorable values which would lead one to expect a small linear expansion and, at the same time, good dissipation of the heat generated in the valve guides, such valve guides have been unacceptable for use in internal combustion engines in motor vehicles because tests showed that the valve guides broke up after a relatively short time.